Structural maturation of the transmissible gastroenteritis coronavirus.

During the life cycle of the transmissible gastroenteritis coronavirus (TGEV), two types of virus-related particles are detected in infected swine testis cells: large annular viruses and small dense viruses. We have studied the relationships between these two types of particles. Immunoelectron microscopy showed that they are closely related, since both large and small particles reacted equally with polyclonal and monoclonal antibodies specific for TGEV proteins. Monensin, a drug that selectively affects the Golgi complex, caused an accumulation of large annular viral particles in perinuclear elements of the endoplasmic reticulum-Golgi intermediate compartment. A partial reversion of the monensin blockade was obtained in both the absence and presence of cycloheximide, a drug that prevented the formation of new viral particles. After removal of monensin, the Golgi complex recovered its perinuclear location, and a decrease in the number of perinuclear large viral particles was observed. The release of small dense viral particles into secretory vesicles and the extracellular medium was also observed, as was a partial recovery of infectivity in culture supernatants. Small viral particles started to be seen between the third and the fourth Golgi cisternae of normally infected cells. All of these data strongly indicate that the large annular particles are the immature precursors of the small dense viruses, which are the infectious TGEV virions. The immature viral particles need to reach a particular location at the trans side of the Golgi stack to complete their morphological maturation.